1. Field of the Invention
The present invention relates to a thermal fixing device, for thermally fixing image developing material, such as toners, onto recording medium, such as sheets of paper.
2. Description of the Related Art
The thermal fixing device is employed in image recording apparatuses, such as printers, copy machines and facsimile machines, of electrophotographic type. The thermal fixing device receives an image recording medium, such as a paper, onto which image developing material, such as toners, has been transferred, and thermally fixes the image developing material onto the image recording medium. The thermal fixing device is constructed from a combination of a heat roller and a pressure roller. Conventionally, the heat roller has a heater, such as a halogen lump, installed therein. The pressure roller is pressed toward the heat roller to form a nip portion, at a region where the heat roller and the pressure roller are in abutment contact with each other. A sheet of paper, onto which a toner image has been transferred, is inserted between the heat roller and the pressure roller, at the nip portion. The nip portion should be supplied with a sufficient amount of heat, a proper amount of pressure force, and a proper amount of adhesibility to thermally fix toners onto the sheet of paper. Accordingly, the heat roller is constructed from a combination of: a fixedly mounted heater; and a cylindrical hollow roller provided rotatably around the heater and formed from material of high thermal transmission characteristics, such as aluminum or stainless steel. The pressure roller is made from heat resistant material such as silicon rubber.
In order to maintain the temperature at the outer surface of the heat roller to be fixed to a proper value, a temperature sensor is provided in the vicinity of the heat roller for detecting temperature of the heat roller. The heater is controlled, dependently on the detected results, to generate a controlled amount of heat. When the thermal fixing device gets out of order and it becomes impossible to control the heat amount, there is a possibility that heat will be excessively generated to damage components located in the vicinity of the thermal fixing device. A temperature fuse is therefore provided in the vicinity of the heat roller. The fuse will be blown by melting, at the abnormally high temperature, to turn off the power source of the device.
The heat roller and the pressure roller constituting the thermal fixing device and the temperature sensor, the temperature fuse, and the like which are located around the thermal fixing device are subjected to high temperature, during, before and after the fixing operation attained by the thermal fixing device. Parts, such as bearings, for supporting these components are therefore made from material, such as resin or metal, of high heat resistant characteristics. Especially, the parts, such as bearings, for supporting the rotating portion of the heat roller are required to have high heat resistant characteristics.
When a sheet of paper is jammed during the fixing operation, an operator has to remove the paper which is located in the vicinity of the thermal fixing device. In order not to injure the operator even when the operator erroneously touches the thermal fixing device, the parts constituting the heat roller and the like are covered with a protection member such as a cover.
In the conventional thermal fixing device having the above-described structure, the heater heats not only the portion of the heat roller contacting the sheets of paper but also the portion of the heat roller not contacting the sheets or paper. The heater heats also the various components, such as the bearings for supporting the heat roller, and heats even atmospheric air around the heater. Accordingly, the thermal fixing device is entirely heated to a high temperature. In order to protect the operator against the thus heated thermal fixing device, the thermal fixing device is entirely covered with a heat insulating material. Accordingly, the structure of the thermal fixing device becomes complicated and large.
Furthermore, in order to protect various components, such as a photosensitive member, a developing device and an image scanner, located in the image recording apparatus, against the heat generated from the thermal fixing device, these components are located apart from the thermal fixing device, a heat insulating material is provided between the thermal fixing device and these components, or a ventilation fan is mounted for discharging the heated atmospheric air outside from the image recording device. The entire structure of the image recording apparatus therefore becomes complicated and large.
When the heater is turned on, heat generated by the heater is transmitted to the inside of the heat roller and raises the temperature of the entire part of the heat roller. It is noted that a long period of time is required until the heat roller is entirely heated to a temperature sufficiently high to fix toners onto sheets of paper. It is impossible to perform the fixing operation during this long waiting time period.
Power consumed during this waiting time period and power consumed for generating heat which is not used for fixing toners but which is discharged outside in vain are relatively large.
In order to shorten the waiting time period, some kinds of thermal fixing devices are preheated to always maintain the temperature of the heat roller to be fixed. In this case, however, a large amount of power is consumed for this preheating operation.
The conventional fixing device can perform high quality fixing operation onto copy sheets and plain papers. However, the conventional fixing device fails to perform high quality fixing operation onto envelopes, sheets of paper having highly rough surfaces, thick sheets of paper, and the like, because large-volume air layers contained in these kinds of papers serve as heat insulation layers. In order to fix toners onto these kinds of sheets of paper with high quality, the fixing device should be supplied with a larger amount of fixing energy, relative to the copy sheets and the plain papers. However, when the fixing device is supplied with energy large enough to fix toners onto the thick papers and the like, if the copy sheet or the like is erroneously transported to the fixing device, the temperature of the roller surface excessively rises.
In some other thermal fixing devices, the respective components constituting the fixing device are designed to have low heat capacity, in order to enhance the heat efficiency of the thermal fixing device and in order to shorten the waiting time period required until start of fixing. For example, the cross-sectional area of the heat roller is made small. In this case, however, the heat transmission is deteriorated in the lengthwise or axial direction of the heat roller, as a result of which heat staying partly in the heat roller is not transmitted well in the lengthwise direction. It therefore becomes liable that the temperature excessively rises at the portions of the heat roller and the pressure roller that do not contact the sheets of paper.